This application claims priority to Japanese Patent Application No. 2001-346849 filed on Nov. 13, 2001.
1. Field of the Invention
The present invention relates to methods and systems for the observation, analysis, and evaluation of thin film samples or fine particles, and more specifically, the present invention relates to systems and methods for analyzing, at multiple stages driving fabrication, electronic devices and/or micro devices such as semiconductor devices, liquid crystal devices, magnetic head devices, that require observation and analysis of not only surfaces of an observation subject but also inner cross sections near the surface thereof.
2. Description of the Background
As a means for obtaining information on the elemental composition of a sample, it is known to detect x-rays generated as a result of electron beam irradiation. X-rays have two components: characteristic radiations with energy specific to the elements comprising the sample, and continuous x-rays whose energy is shorter than the energy of the incident electron beam as a result of Bremsstrahlung radiation. By analyzing the energy spectrum of x-rays, it is possible to find out the elemental composition of the sample. JP-A-68060/1980 discloses a collimator having an opening, which is smaller than an x-ray detector, for taking in x-rays from a sample (Related Art 1). FIG. 21 shows a schematic view of a device that has been improved from the structure of the Related Art 1.
The improved structure works as follows. Electron beam 8 is irradiated onto a sample 22, which emits x-rays due to the irradiation. An x-ray detector 16 having an x-ray detecting element 161 provided above in a slanting direction of the sample 22, and a collimator 162 for restricting the optical path of x-rays disposed between the x-ray detecting element and the sample, detects the x-rays emitted from the sample.
FIG. 23 is a schematic enlarged view showing around the sample 22 in an in-lens electron microscope shown in FIG. 22. A micro sample 22 is introduced into a space interposed between an upper magnetic pole 707 and a lower magnetic pole 708 comprising an objective lens. A mesh 26 holds the micro sample 22, which is attached to a sample holder 706 therethrough. X-rays 401, secondary electrons 301, and reflected electrons 205 are generated as an electron beam 8 irradiates the sample 22. When the micro sample 22 is analyzed for its elemental structure by detecting x-rays thereof, it is ideal if the measured x-rays are only the x-rays 401. In actuality, however, various aspects generate x-rays as described below.
A part of the reflected electrons 205 generates x-rays 402 with an energy having no bearing to the sample itself by colliding with a surface of the upper magnetic pole 707. Transmitted electrons 201 and 202 passing. through the sample 22 collides with the sample holder 706 and the lower magnetic poles 708 so as to generate X-rays 403 and 406. Reflected electrons 207 generated by the transmitted electron 202 being scattered at the lower magnetic pole 708 collide with the sample holder 706 so as to generate x-rays 407. X-rays generated by the transmitted electron 202 colliding with the lower magnetic pole are incident on the sample holder 706 so as to generate x-rays 405. Although not shown in any of the figures, there are other x-rays generated by reflected electrons and transmitted electrons colliding with other parts of the sample 22 and the mesh 26.
These reflected x-rays are called xe2x80x9cbackground x-raysxe2x80x9d because they are not generated from the sample. The background x-rays cause to deteriorate accuracy of the elemental analysis. A collimator 162 shown in FIG. 21 is provided to reduce the above-described background x-rays entering into the x-ray detecting element 161 as much as possible.
Another method for reducing the background x-rays is disclosed in xe2x80x9cPrinciples of Analytical. Electron Microscopyxe2x80x9d, edited by David C Joy et al., p.p. 131-135, 1986, Plenum Press, New York (Related Art 2). In this example, as shown in FIG. 24, surfaces of upper 707 and lower 708 magnetic poles have plates 501 and 502, respectively of a light elemental material having a hole for transmitting the electron beam, whereby the reflected electron 205 and transmitted electron 202 from a sample 22 collide with a light elemental plate 501, 502 rather than directly colliding with the magnetic poles 707, 708 mainly formed of Fe. In this way, the number of reflected electrons at the magnetic poles, the energy characteristics of the x-rays, and the amount of continuous x-rays are reduced, thus enabling a reduction in the background x-rays as a result thereof.
JP-A-261894/1997 discloses a method for reducing background x-rays generated by reflected electrons and transmitted electrons colliding with places other than an observation point on a sample (Related Art 3). In this method, when creating an observation surface in the form of a thin film from the sample, the observation surface is formed so as to be inclined with respect to a side face of the sample that is not made into a thin film. A carbon film covers a surface of the sample other than the observation point. There is also a method using a sample stage covered by carbon.
The above-described conventional methods have at least the following problems. The method of Related Art 1 prevents x-rays generated from a non-sample portion from entering into a detecting element by providing a collimator 162 between the x-ray detecting element 161 and the sample 22. In case of using a thin film sample, such a method is not effective with respect to x-rays generated by transmitted electrons that pass through the sample and collide with a sample stage immediately below the sample. Moreover, when a narrow collimator 162 is provided to limit x-rays only from an electron beam irradiation point on the sample 22, a distance between the detecting element 161 and the sample 22 has to be relatively long, thus lowering detection efficiency due to lack of a proper detection angle.
Furthermore, the x-ray detector 161 must be accurately placed with respect to the sample 22. Thus, displacement of the sample 22 would result in decreased detection sensitivity.
In Related Art 2, a light elemental material covers the surfaces of upper and lower magnetic poles of the objective lens. The method is effective when a transmission electron microscope that has high accelerating voltage of the electron beam and when a thin sample with a thickness equal to or less than 100 nm is used because of a decrease in the scattering of electron beams transmitted through the sample. However, when a general scanning electron microscope, or a sample with a thickness of 100 nm or more is used, the electron beam transmitted through the sample has a greater scattering angle. Thus, background x-rays due to the collision of electron beams with portions other than the parts of light elemental material covering the magnetic poles or a sample stage increase, thereby deteriorating detection accuracy.
Even in a processing method of a micro sample as shown in Related Art 3, background x-rays generated by a collision of the electron scattered as described above with portions other than a sample stage or a sample chamber are not considered. Thus, the ratio between a signal and a background noise becomes undesirable, and measurement accuracy is also lowered because x-rays from portions other than measurement positions are also detected. The x-rays generated when scattered electrons collide with a carbon film deposited on a sample would be reduced compared to the case without the carbon film, although the degree of reduction is not sufficient. The method for forming a carbon film on a part of the micro sample requires deposition equipment, and the deposition becomes necessary every time a sample is made. Thus, the production of a sample becomes increasingly complicated and requires additional time.
Where a carbon film covers a sample stage, signals from elements comprising the sample stage may be reduced, but the carbon film emits characteristic x-rays and continuous x-rays. Therefore, the signal to noise ratio cannot be greatly improved with this method.
As described above, potential problems in elemental analysis by x-rays detection are mentioned. An additional problem is that the electrons scattered by passing through the sample generate reflected electrons by colliding with other parts of the apparatus, and the reflected electrons are incident upon portions other than the measurement point of the sample so as to generate secondary electrons, thereby making it different from an original secondary electron image.
In view of the above, an object of the present invention is to provide an apparatus and a method for observing samples that is capable of making vertical cross-sectional observation of an inner cross section of a target sample, and of high-resolution, high accuracy, and high through-put observation/analysis, and moreover, to provide an apparatus and method capable of observing and analyzing a sample without failure or degradation due to an exposure to the air.
An apparatus for observing samples according to at least one embodiment is characterized in that, the apparatus comprises: an electron beam irradiating optical system having an electron source, a lens for focusing the electron beam, an electron beam scanning deflector; a sample stage upon which a sample is placed; and an electron beam detector for detecting electrons generated by the sample by irradiating the electron beam on the sample and/or an x-ray detector for detecting x-rays, in which the observation is performed by placing a piece (shielding piece) having a hole and made from at least a light elemental material to the sample stage behind the sample and by irradiating the electron beam. With this structure, the electron beam that is incident on parts other than the sample after being transmitted through a thin film sample is incident on a piece of light elemental materials, thereby reducing the number of back scattered electrons and continuous x-rays generated therefrom.
Thus, it is possible to detect the characteristic x-ray spectrum peak of the element included in the sample with a high signal to noise ratio with respect to the continuous x-ray spectrum on the background. It is also possible to reduce the influence of reflected electrons generated by the electron beam transmitting through the sample and colliding with other parts or secondary electrons and transmitted electrons generated by reentering the micro sample. It is thus possible to make a high spatial resolution observation and an accurate measurement of the secondary electrons and reflected electrons. Moreover, the sample does not have to be taken out from the apparatus, thus observation and analysis can be quickly made.
In an elemental analysis by characteristic x-ray detection when irradiating with a charged particle beam, by making a thin film of the micro sample, it is possible to decrease the x-rays generation area due to scattering of the charged particle beam in the sample. Moreover, by introducing a piece of light elemental material, it is possible to reduce x-rays generated by incidence of the electron beam on places other than the micro samples after being transmitted through the target sample, or x-rays generated by back scattered electrons from these other places reentering to the target sample, thereby enabling a high spatial resolution elemental analysis.
Thus, it is possible to provide an apparatus for observing a sample that is capable of observing and analyzing an inner cross section of the target sample with high-resolution, high accuracy, and in a short period of time.
In at least one preferred embodiment, an apparatus for observing a sample is characterized in that the apparatus includes: a focused ion beam irradiation optical system having an ion source, a lens for focusing the ion beam, and an ion beam scanning deflector; an electron bean irradiating optical system having an electron source, a lens for focusing the electron beam, and an electron beam scanning deflector; and a sample stage upon which a sample is placed. There is preferably provided a means for separating a second sample from the sample by using the focused ion beam, a manipulator for extracting the second sample, and either or both of an electron beam detector for detecting electrons generated from the sample by irradiating the electron beam to the micro sample, and/or an x-ray detector for detecting x-rays. The function includes observing the second sample by the electron beam by placing a piece (shielding piece) having a hole and made from at least a light elemental material behind the extracted second sample on the sample stage or between the sample stage and the second sample.
Accordingly, it is possible to provide a high accuracy observation with high spatial resolution as described above. In addition, it is possible to provide an apparatus for observing samples that is capable of observing and analyzing the sample in a short time period because the sample does not have to be taken out from the apparatus.
An apparatus for observing a sample may be characterized in that the manipulator for extracting the second sample separated from the introduced sample has a manipulator driver for driving the manipulator independent from the sample stage, and a function of varying an irradiation angle of a charged particle beam for observing the second sample while holding the second sample by the manipulator.
Accordingly, the sample including a portion for observation can be moved with respect to the scanning electron optical system or the focused ion beam irradiation optical system while remaining attached to the manipulator, thereby enabling the disposition of the micro sample to a place with a more favorable observation resolution. It is possible to freely select an observation orientation of an inner cross section of the target sample. Thus, it is possible to provide an apparatus for observing a sample that is capable of making high-resolution observations and high accuracy measurement and evaluation of a film thickness, an implantation status, and a shape and size of etched or planarized portion by observing the cross section perpendicularly. Because the sample does not need to be taken out from the apparatus, observation and analysis can be made rapidly.
In at least one embodiment, an apparatus for observing a sample is characterized in that a second manipulator having a light element piece on a tip thereof introduces the light element piece into a space behind the observation sample extracted and supported by the first manipulator, i.e., a space opposite from a component generating the charged particle beam for observation.
Accordingly, it is possible to reduce the influence of back scattered electrons generated by an electron beam incident on portions other than the sample after being transmitted through the sample, thereby enabling to make high-resolution observation. Moreover, because the sample does not have to be taken out from the apparatus, observation and analysis can be made in a reduced time period.
In an elemental analysis by characteristic x-ray detection while irradiating the charged particle beam, by making a thin film of the micro sample, it is possible to decrease the x-ray generation area due to scattering of the charged particle beam in the sample. Moreover, by introducing a piece of light elemental material, it is possible to reduce x-rays generated by incidence of the electron beam on places other than the micro samples after transmitting through the target sample, or x-rays generated by back scattered electrons from these other places returning to the target sample, thereby enabling a high spatial resolution elemental analysis.
An apparatus for observing samples according to an additional aspect of the invention is characterized in that the apparatus includes a second sample stage having a light element piece of a first aspect attached to the sample stage. The sample to be observed is placed proximate to the light element piece.
Accordingly, a high-resolution observation is achieved by the above-described reasons, and a rapid observation and analysis are also possible. By fixing the extracted sample to a part of the second sample stage, it is possible to avoid the effects of vibration during the sample observation.
An apparatus for observing samples according to an additional aspect is characterized in that the apparatus includes a third sample stage having a function for varying irradiation position and angle of charged beam for observation for the sample. The apparatus preferably includes a light element piece that is driven independently from the sample stage of the first aspect and places a sample including an observation target extracted from the sample.
Accordingly, it is possible to provide an apparatus for observing samples that is capable of observing and analyzing a target inner cross section of the sample with a high-resolution in a short period of time. A plurality of samples can be fixed on the second sample stage by one manipulator by separating the extracted sample from the manipulator, such that time for cross-sectional observation and elemental analysis is decreased. Moreover, because the sample is fixed on the second sample stage by being separated from the manipulator, it is possible to avoid any vibration problem during sample observation.
An additional apparatus for observing samples is characterized in that the shielding piece according to the above aspects includes a hole, and is placed immediately behind the sample. The ratio of the distance between the sample and an upper surface of the hole to the diameter of the hole is preferably no more than ⅕.
Accordingly, the electron beam transmitting through the sample is made incident on the hole of the light element piece, and the incident electron collides with a side and bottom of the hole. The majority of the electrons are scattered toward the hole bottom as they collides with the side surface, thus enabling a significant reduction in the number of electrons returning to the surface. Moreover, x-rays generated by the collision of elections inside the hole and incident to the hole after being transmitted through the sample reaches the x-ray detector after passing though the light element piece. Because the x-rays decay as they pass, it is possible to reduce the influence of x-rays generated in the hole. Accordingly, it is possible to provide an apparatus for observing samples that is capable of making accurate and high-resolution observations.
An additional apparatus for observing samples is characterized in that a heavy elemental material covers the shielding piece having the above-described hole. Accordingly, x-rays generated by collisions inside the hole after being transmitted through the sample decay in the shielding piece of the light elemental material, and further decay in the heavy elemental material having stronger decaying ability that covers outside of the shielding piece, thereby enabling a reduction in x-rays generated in the hole that would be detected by the x-ray detector. Thus, it is possible to provide an apparatus for observing samples that is capable of making accurate and high-resolution observation.
An additional apparatus for observing samples is characterized in that the light element piece is formed of beryllium, carbon or a compound of carbon and beryllium. Generally, when electrons collide with a substance of a certain element, x-rays called xe2x80x9ccharacteristic x-raysxe2x80x9d specific to the element of the substance and continuous x-rays, i.e., Bremsstrahlung radiations, are generated. The characteristic x-rays are observed as a spectrum peak so that information about the element contained in the sample can be obtained from the peak. Therefore, such a peak should not be generated from portions other than an observation target of the sample. As described above, in the present invention, an electron beam transmitting through the thin film sample is preferably incident into a hole of a light elemental material located immediately behind the thin film sample so as to generate x-rays inside the hole. If the energy of the x-rays is reduced, it is more likely that the x-rays are absorbed and decayed in the substance. Therefore, by comprising the piece of a light element, such as beryllium or carbon, background x-rays can be reduced greatly. The energy of the x-rays of beryllium is 110 eV, which is not detectable with a conventional semiconductor detector; thus, it is desirable to use the beryllium.
Compared to the case where the electron collides with a wafer comprised mainly of silicon or a piece made of metal material comprising a sample holder, continuous x-rays generated from a substance made of beryllium or carbon by the electron collision after transmitting the sample therewith is less than ⅕ this amount. It is thus possible to reduce continuous x-rays, and the number of reflected electrons can also be reduced. Accordingly, it is possible to provide an apparatus for observing samples with a high-resolution and high accuracy.
An additional apparatus for observing samples is characterized in that a thickness of the light element piece is thicker than a depth of penetration of the electron beam. Accordingly, the electron beam transmitted through the sample does not transmit through the light element piece, but is absorbed thereby. Therefore, it is possible to provide an apparatus for observing samples that is capable of making observation with high-resolution and accuracy.
An additional apparatus for observing samples is characterized in that the light element piece is grounded. Accordingly, the light element piece is not charged, thereby enabling an apparatus for observing samples that is capable of making observations with high-resolution and accuracy.
A method for observing samples with an apparatus is characterized in that, the apparatus includes: an electron beam irradiating optical system having an electron source, a lens for focusing the electron beam, an electron beam scanning deflector; a sample stage for placing a sample; and a detector for detecting x-rays and electrons generated from the sample by irradiating the electron beam to the sample. The sample is observed by irradiating the electron beam while setting a piece (shielding piece) that at least includes a light elemental material behind the sample. Accordingly, it is possible to reduce the influence of x-rays and back scattered electrons generated by the electron beam transmitting through the sample that are incident on portions other than the sample. Thus, it is possible to provide a method for observing samples that can observe and analyze samples accurately and with greater sensitivity.
Moreover, a method for observing samples with an apparatus is characterized in that, the apparatus includes: a focused ion beam irradiation optical system having an ion source, a lens for focusing the ion beam, and an ion beam scanning deflector; an electron beam irradiating optical system having an electron source, a lens for focusing the electron beam, and an electron beam scanning deflector; and a sample stage upon which a sample is placed. The observing method includes a function for separating a second sample from the sample by using the focused ion beam, a manipulator for extracting the second sample, a detector for detecting x-rays and electrons generated from the second sample by irradiating the electron beam to the micro sample, and for observing the second sample by the electron beam while setting a piece (shielding piece) made from at least a light elemental material behind the extracted second sample.
Accordingly, it is possible to freely align an observation surface of the sample extracted with respect to the direction of the electron beam for observation without exposing the sample to the air, and to reduce an influence of x-rays and back scattered electrons generated by the electrons transmitted through the sample colliding with portions other than the sample. Thus, it is possible to provide a method for observing samples that can observe and analyze an inner cross section of the sample with high-resolution and in a short period of time. In particular, by applying the method to the semiconductor wafer, it can be used for a process check at various stage of semiconductor production, thereby contributing to improved yield percentages of production by quick quality control and early detection of device defects.